The present invention concerns a process for catalytic cracking of hydrocarbons using a sepiolite-containing catalyst which has both a large pore system and good attrition resistance.
Catalytic cracking systems employ catalyst in a moving bed or a fluidized bed. Catalytic cracking is carried out in the absence of externally supplied molecular hydrogen, and is, for that reason, distinctly different from hydrocracking, in which molecular hydrogen is added in processing. In catalytic cracking, an inventory of particulate catalyst is continuously cycled between a cracking reactor and a catalyst regenerator. In a fluidized catalytic cracking (FCC) system, a stream of hydrocarbon feed is contacted with fluidized catalyst particles in a hydrocarbon cracking zone, or reactor, at a temperature of about 425.degree.-600.degree. C., usually 460.degree.-560.degree. C. The reactions of hydrocarbons at the hydrocarbon stream at the elevated operating temperature result in deposition of carbonaceous coke on the catalyst particles. The resulting fluid products are separated from the coke-deactivated, spent catalyst and are withdrawn from the reactor. The coked catalyst particles are stripped of volatiles, usually by means of steam, and passed to the catalyst regeneration zone. In the catalyst regenerator the coked catalyst is contacted with a predetermined amount of molecular oxygen. A desired portion of the coke is burned off the catalyst, restoring catalyst activity and simultaneously heating the catalyst to, e.g. 540.degree.-815.degree. C., usually 590.degree.-730.degree. C. Flue gas formed by combustion of coke in the catalyst regenerator may be treated for removal of particulates and conversion of carbon monoxide, after which it is normally discharged into the atmosphere.
Sepiolite is a naturally occurring clay-type mineral, which often has a lath-shaped or fibrous morphology. It is a hydrated magnesium silicate, also known as meershaum. Discussions of sepiolite are found in the books Clay Mineralogy, by R. E. Grim, published by McGraw-Hill (2d Ed., 1968), and The Electron-Optical Investigation of Clays, J. A. Gard, Ed., puyblished by the Mineralogical Society (Great Britain, 1971). Sepiolite can be formed synthetically by known techniques. Although sepiolite can occur in forms other than laths, rods or fibers, only the lathe-type sepiolite is suitable for use in providing the essential sepiolite rods used in the catalyst of the invention, as discussed below.
The use of faujasite type zeolites in catalytic cracking is well known. The use of crystalline aluminosilicate zeolites having uniform pore openings in the range from 5.5-7.0 Angstroms and maximum cage dimensions of 5.5-7.0 Angstroms for catalytic cracking is also known. For example, U.S. Pat. Nos. 3,758,403, 3,849,029 and 3,856,659 all suggest the use of the zeolite ZSM-5 in a dual-zeolite catalyst, along with a conventional crystalline aluminosilicate having larger pore openings and cages, such as a Y-type zeolite. U.S. Pat. No. 3,894,934 suggests the use of a carbon monoxide combustion-promoting component in conjunction with ZSM-5 and a large-pore-size crystalline aluminosilicate. The use of ZSM-5 containing active catalytic metal values to catalyze aromatics alkylation is suggested in U.S. Pat. No. 3,953,366. ZSM-5-containing catalysts are also discussed in U.S. Pat. Nos. 3,702,886 and 3,926,782. Crystalline silicates are described in U.S. Pat. Nos. 4,061,724 and 4,073,865.